Studies of the implementation of efficacious HIV prevention interventions are relatively rare, especially in international settings, but are critically important since they have the potential to increase the public health impact of interventions by improving uptake and sustainability. Few implementation studies have focused on provider and organizational factors (e.g., acceptability of program to staff, extent of organizational support for change) that may influence uptake and fidelity to core components of interventions. We propose to study the implementation of an efficacious intervention to reduce STIs among female sex workers (FSWs). In a previous study we developed a brief 35-minute intervention, Mujer Segura, which was delivered by community-based counselors; this intervention significantly reduced STI/HIV incidence by 40%. The Mexican federal government supports the scaling-up of this intervention across the country, and has encouraged us to formally evaluate this process. We propose to test an implementation model for transporting Mujer Segura into Community Based Organizations (CBOs) in Mexico. We have partnered with MexFam, a non-governmental organization that has CBOs throughout Mexico. Our specific aims are to: 1) Determine if our implementation model can achieve high levels of intervention fidelity and provider competency in the context of a large-scale implementation effort; 2) Characterize the relationship between individual provider characteristics and organizational factors and determine their impact on the implementation of Mujer Segura using a mixed-methods (quantitative and qualitative) approach; 3) Determine whether the implementation of Mujer Segura by CBOs is associated with decreased sexual risk behaviors among Mexican FSWs over a six-month period (e.g., increased condom use with clients, reductions in STI incidence); and 4) Determine whether improvements in sexual risk behaviors among Mujer Segura FSWs are associated with variations in intervention fidelity and counselor competency. This project will collect systematic data on the implementation process and test a model that could generalize to future implementation efforts involving Mujer Segura and other evidence-based interventions in resource- constrained settings. Our binational research team is poised to conduct this research, which will provide critical insights into barriers associated with implementing interventions in CBOs.